Immunotoxins combine the activity of a powerful toxin with an antibody capable of delivering the conjugate to a specific target cell. These compounds have proven effective in the in vitro elimination of a number of cell types, including leukemia cells and the bone marrow cells implicated in graft vs. host disease. Immunotoxins may be an effective way of controlling a number of diseases attributable to a population of antigenically distinct cells. The in vivo use of immunotoxins is currently limited by an unacceptably high level of non-specific toxicity. This probably results from the ability of the toxic component to enter the cytosol without antibody help. The goal of this research is to develop immunotoxin components that retain their intracellular toxicity, but are incapable of transmembrane relocation unless bound to antibody. Five ribosome-inactivation proteins capable of functioning as immunotoxin components will be enzymatically or chemically cleaved into smaller peptides. It is hoped that by this process functional domains of the toxins can be found that retain the ability to catalytically inactivate ribosomes, but are incapable of self-directed entry into the cytosol. The activity of these peptides will be tested in vitro in whole cell and cell-free assays of protein synthesis, as well as in vivo where the LD50 of active peptides will be compared to that of the intact toxin. The amino acid sequence of active peptides will also be determined.